The present invention relates to organic electroluminescent devices.
Organic electroluminescent devices are a class of opto-electronic devices where light emission is produced in response to an electrical current through the device. (For brevity, EL, the common acronym for electroluminescent, is sometimes substituted.) The term organic light emitting diode or OLED is also commonly used to describe an organic EL device where the current-voltage behavior is non-linear, meaning that the current through the EL device is dependent on the polarity of the voltage applied to the EL device. In this embodiment, the term EL and EL devices will include devices described as OLED.
Organic EL devices generally have a layered structure with an organic luminescent medium sandwiched between an anode and a cathode. The organic luminescent medium usually refers to an organic light emitting material or a mixture thereof in the form of a thin amorphous or crystalline film. Representatives of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, xe2x80x9cDouble Injection Electroluminescence in Anthracenexe2x80x9d, RCA Review, Vol. 30, pp. 322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. In these prior arts, the organic luminescent medium was formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terpheyls, quarterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents. The organic luminescent medium was present as a single layer having a thickness much above 1 micrometer. The voltage required to drive the EL devices was as much as a few hundreds volts, thus the luminous efficiency of these EL devices was rather low.
In commonly-assigned U.S. Pat. No. 4,356,429, Tang further advanced the art of organic EL device by disclosing a bi-layer EL device configuration. The organic luminescent medium in this bi-layer configuration comprises of two extremely thin layers of organic film ( less than 1.0 micrometer in combined thickness) sandwiched between the anode and cathode. The layer adjacent to the anode, termed the hole-transport layer, is specifically chosen to transport predominantly holes only in the EL device. Likewise, the layer adjacent to the cathode is specifically chosen to transport predominantly electrons only in the EL device. The interface or junction between the hole-transport layer and the electron-transport layer is referred to as the electron-hole recombination zone where the electron and hole recombine to produce electroluminescence with the least interference from the electrodes. This recombination zone can be extended beyond the interface region to include portions of the hole-transport layer or the electron-transport layer or both. The extremely thin organic luminescent medium offers reduced electrical resistance, permitting higher current densities for a given voltage applied on the EL device. Since the EL intensity is directly proportional to the current density through the EL device, this thin bi-layer construction of the organic luminescent medium allows the EL device to be operated with a voltage as low as a few volts, in contrast to the earlier EL devices. Thus, the bi-layer organic EL device has achieved a high luminous efficiency in terms of EL output per electrical power input and is therefore useful for applications such as flat-panel displays and lighting.
Commonly-assigned Tang U.S. Pat. No. 4,356,429 disclosed an EL device formed of an organic luminescent medium including a hole transport layer containing 1000 Angstroms of a porphyrinic compound such as copper phthalocyanine, and an electron transport layer of 1000 Angstroms of tetraphenylbutadiene in poly(styrene). The anode was formed of a conductive indium-tin-oxide (ITO) glass and the cathode was a layer of silver. The EL device emitted blue light when biased at 20 volts at an average current density in the 30 to 40 mA/cm2 range. The brightness of the device was 5 cd/m2.
Further improvements in the bi-layer organic EL devices were taught by commonly-assigned Van Slyke et al U.S. Pat. No. 4,539,507. Van Slyke et al realized dramatic improvements in EL luminous efficiency by substituting the porphyrinic compounds of Tang in the hole-transport layer with an amine compound. With an aromatic tertiary amine such as 1,1-bis(4-di p-tolylaminophenyl)cyclohexane as the hole-transport layer and an electron transport layer of 4,4xe2x80x2-bis(5,7-di-t-pentyl-2-benzoxazolyl)-stilbene, the EL device was capable of emitting blue-green light with a quantum efficiency of about 1.2% photon per injected charge when biased at about 20 volts.
The use of aromatic amines as the material for the hole-transport layer in organic EL devices has since been generally recognized as numerous prior arts have disclosed the utility of various classes of amines in enhancing the EL device performance. Improvements in the hole-transport material parameters include higher hole transport mobility, more amorphous structures, higher glass transition temperature, and better electrochemical stability. Improvements in the organic EL devices with these improved amines include higher luminous efficiency, longer operational and storage life, and a greater thermal tolerance. For example, the improved arylamine hole transport materials have been disclosed in commonly-assigned U.S. Pat. No. 5,061,569 by VanSlyke et al. A series of aromatic amines with glass transition temperature as high as 165xc2x0 C. designed for high temperature EL devices has been disclosed in commonly-assigned U.S. Pat. No. 5,554,450 by Shi et al. A xcfx80-conjugated starburst molecule 4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylamino) triphenylamine (m-MTDATA), which forms a stable amorphous glass and functions as an excellent hole transport material, was disclosed in U.S. Pat. No. 5,374,489 by Shirota et al.
The use of organic compounds outside the aromatic amines class for the hole-transport layer in organic EL devices is not common, given the well-known hole-transport properties of the aromatic amines. However, there is a significant disadvantage of using aromatic amines as the hole-transport layer in the bi-layer EL device. Since amines are generally strong electron donors, they can interact with the emissive materials used in the electron-transport layer, resulting in the formation of fluorescence quenching centers and a reduction in the EL luminous efficiency.
It is an object of the present invention to provide organic compounds outside the class of aromatic amines as the hole transport layer in organic EL devices, which result in enhanced EL performance.
This object is achieved in an organic electroluminescent device, comprising:
an anode and cathode and therebetween a hole transport layer; and an electron tranport layer disposed in an operative relationship with the hole transport layer; wherein:
the hole transport layer includes at least an aromatic hydrocarbon or fused aromatic hydrocarbon containing at least 20 carbon atoms and having an ionization potential greater than 5.0 eV.
Representative examples of the hole transport layer material include:
a) Anthracene derivatives of formula I: 
xe2x80x83wherein:
substituents R1, R2, R3 and R4 are each individually hydrogen, or alkyl of from 1 to 24 carbon atoms; aryl or substituted aryl of from 5 to 28 carbon atoms; or heteroaryl or substituted heteroaryl of from 5 to 28 carbon atoms; or fluorine, chlorine, bromine; or cyano group;
b) Arylethylene and arylacetylene derivatives of formulae II, III, IV, and V: 
xe2x80x83wherein:
n is integer from 1 to 6;
substituents R1, R2, R3, R4, R5 and R6 are each individually hydrogen, or alkyl of from 1 to 24 carbon atoms; aryl or substituted aryl of from 5 to 28 carbon atoms; or heteroaryl or substituted heteroaryl of from 5 to 28 carbon atoms; or fluorine, chlorine, bromine; or cyano group; and
c) polyphenyl hydrocarbons of formula VI: 
xe2x80x83wherein:
n is integer from 1 to 6;
substituents R1, R2 and R3 are each individually hydrogen, or alkyl of from 1 to 24 carbon atoms; aryl or substituted aryl of from 5 to 28 carbon atoms; or heteroaryl or substituted heteroaryl of from 5 to 28 carbon atoms; or fluorine, chlorine, bromine; or cyano group.
Aromatic hydrocarbon or fused aromatic hydrocarbon that are used in the hole transporting layer have the feature that do not need to include alkylamino- or arylamino-moieties;
The aromatic hydrocarbon or fused aromatic hydrocarbon in accordance with the present invention have an ionization potential larger than 5.0 eV.
Quite unexpectedly, it has been found that hole transport layers in accordance with the present invention that have an ionization potential greater than 5.0 eV effectively work with the electron transport layer or an emissive layer or an electron transport layer which also functions as an emissive layer to provide a highly efficient electroluminescent device.